1. Preheat grill to high heat.
2. Combine first 9 ingredients in a small bowl; stir with a whisk. Chill buttermilk mixture until ready to serve.
3. Lightly coat chicken with cooking spray. Combine oil, onion powder, cumin, 1/2 teaspoon salt, and chipotle; rub evenly over chicken. Coat corn and onion with cooking spray. Arrange chicken, corn, and onion on grill rack; grill 8 minutes or until done, turning chicken and onion once and corn occasionally. Remove from grill; let stand 5 minutes. Cut corn kernels from cobs.
4. Slice chicken. Arrange 1 breast on each of 4 plates. Arrange 2 yellow and 2 red tomato slices on each plate. Top each serving with 1/4 cup cherry tomatoes. Divide corn, onion, and avocado evenly among plates. Sprinkle 1/4 teaspoon salt over salads. Drizzle about 1 1/2 tablespoons dressing over each salad.